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Godding, Robert 
  
springback using the finite element method (FEM) is still problematically, because the implemented material and 
friction laws have to be optimized. 
The assessment of the stamping part geometry using tactual coordinate measuring machines is state-of-the-art. This 
time-consuming method is not capable e.g. to detect part shape fluctuations caused by spreading mechanical properties 
of the sheet material. Contactless optical measurement techniques have advantages especially concerning the measuring 
speed as well as the measuring area and therefore have the potential to be applied in-process. 
Here projected light can be used to reproduce the stamping part geometry. In this case the reflection behavior of the 
sheet surface has to be taken into account, especially on uncoated materials. An advantage of the grid projection is that 
the part could be used in the further production process. However, grid projection is not suitable for the strain analysis. 
3 VISIOPLASTIC ANALYSIS 
For the design of new forming tools the knowledge of critical strain conditions in the stamping part is necessary to 
avoid failures, e.g. cracks due to excessive sheet thinning. During the design of the forming tools numerical simulation, 
especially finite element analysis (FEM) is used for the prediction of formability. During the try-out process and in case 
of modifications of process parameters of the forming process (e.g. change of the sheet material) the experimental strain 
analysis using the method of visioplasticity has to be applied. This method permits a statement about the influence of 
the used sheet material, tool design, the blank shape and the tribological conditions on the final part quality. 
Strain analysis in sheet metal forming is usually accomplished by marking a grid of known dimension on the flat sheet 
blank and measuring the deformed grid on the part after the stamping process. For the evaluation of the strain-rates and 
strain-distribution circles or square texture patterns have to be fixed to the surface to identify corresponding points at 
different deformation stages. Following two different methods are described /9/, which have specific advantages and 
disadvantages concerning an automated computer-based evaluation. 
3.1 Circle grid analysis 
In a homogenous forming process circles marked on the undeformed sheet will distort to ellipses of major and minor 
axes, as illustrated in Figure 5. Assuming proportional deformation the principal directions are coincident with these 
axes. Assuming incompressibility the principal strains €, €», 3 can be calculated with the principal elongations 1;, I, 
and the circle diameter do: 
1 
=f] (1) sms Q) q,--(o, *9,) GB) 
In sheet metal forming usually the forming limit diagram (FLD) is plotted as shown in Figure 6. The FLD provides 
information about how much a particular metal can be stretched before necking or fracture occurs. The necking strains 
are obtained under a variety of biaxial forming conditions so that most of the practical stamping conditions are 
duplicated. Therefore, the FLD provides an indication of the material behavior under actual forming conditions. There 
are several forming conditions of particular interest: stretch forming, plane strain, uniaxial tension and deep drawing. 
The forming limit curve (FLC), also shown below, represents the material specific strain limit, which a part can bear 
without fractures. It is plotted on the forming limit diagram to show how close a particular part is to the forming limit 
under the specific strain state. The circle grid analysis is advantageous for finding strains at single measurement points 
in sheet metal components by measuring the length and direction of the principal axes of the ellipse. However, it is less 
convenient for a computer-based determination of strain distributions in whole regions. 
(07-90; Qq,7-29, ¢,=0 9,799, 
a forming limit 
circle ellipse curve (FLC) - 
     
      
l 
plane strain 
  
major strain p, 
P 
M 
nO 
  
  
  
Y 
minor strain q, 
Figure 5: Deformation of a circle in a homogenous field Figure 6: Forming limit diagram (FLD) 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 293